UNIVERSITY   OF   PENNSYLVANIA 

OPTICAL  CONSTANTS  OF  THE 

BINARY  ALLOYS  OF  SILVER  WITH  COPPER 

AND  PLATINUM 


BY 


LOUIS   K.   OPPITZ 


A  THESIS 

PRESENTED  TO  THE  FACULTY  OF  THE  GRADUATE  SCHOOL  IN  PARTIAL 

FULFILLMENT  OF  THE  REQUIREMENTS  FOR  THE  DEGREE 

OF  DOCTOR  OF  PHILOSOPHY 


PRESS  OF 

THE  NEW  ERA  PRINTING  COMPANY 
LANCASTER,  PA. 


UNIVERSITY   OF   PENNSYLVANIA 

OPTICAL  CONSTANTS  OF  THE 

BINARY  ALLOYS  OF  SILVER  WITH  COPPER 

AND  PLATINUM 


BY 

LOUIS   K.   OPPITZ 

*N\ 


A  THESIS 

PRESENTED  TO  THE  FACULTY  OF  THE  GRADUATE  SCHOOL  IN  PARTIAL 

FULFILLMENT  OF  THE  REQUIREMENTS  FOR  THE  DEGREE 

OF  DOCTOR  OF  PHILOSOPHY 


PRESS  OF 

THE  NEW  ERA  PRINTING  COMPANY 
LANCASTER,  PA. 

IQI7 


[Reprinted  from  the  PHYSICAL  REVIEW,  N.  S.,  Vol.  X,  No.  2,  August,  1917.] 


OPTICAL  CONSTANTS  OF  THE  BINARY  ALLOYS  OF  SILVER 
WITH   COPPER  AND   PLATINUM. 


T 


BY  Louis  K.  OPPITZ. 

HISTORICAL  INTRODUCTION. 

HE  first  studies  in  the  optical  constants  of  alloys  were  those  of 
Drude1  who  investigated  three  alloys:  (a)  one  of  18  k.  gold  alloyed 
with  silver,  copper,  and  a  small  quantity  of  iron;  (&)  one  of  copper-nickel; 
and  (c)  Wood's  alloy.  He  made  no  attempt,  however,  to  study  any 
complete  series  of  alloys  related  according  to  some  well  conceived  prop- 
erty. 

Willi  Meier2  measured  the  optical  constants  of  Wood's  alloy  and  those 
of  an  alloy  of  gold  and  silver  of  equal  parts  by  weight.  His  main  interest 
was  in  the  study  of  optical  constants  for  a  series  of  wave  lengths  which 
extended  into  the  ultra-violet  region. 

Bernouilli3  measured  the  optical  constants  of  a  number  of  alloys,  that 
form  solid  solutions;  but  restricted  his  examinations  to  small  concen- 
trations, that  is  to  dilute  solid  solutions.  His  work  includes  a  study  of 
the  optical  constants  of  Ag-Tl,  Ag-Sn,  Cd-Hg,  Cu-Sn  and  Cu-Ni.  The 
main  interest  in  his  work  is  his  method,4  which  consisted  of  the  measure- 
ment of  the  minimum  azimuth  of  restored  polarization. 

Voigt5  has  criticized  the  mathematical  formula  employed  by  Bernouilli 
as  an  illegitimate  approximation. 

Littleton6  was  the  first  to  study  the  variation  of  optical  constants  for 
entire  alloy  series.  He  investigated  the  alloy  series  of  Cu-Ni,  Fe-Mn, 
Ni-Fe,  Ni-Si,  Al-Cu  and  Cu-Fe.  These  alloys  seem  not  to  have  been 
chosen  for  the  purpose  of  studying  group  characteristics. 

In  1912,  Eckhardt7  investigated  a  series  of  gold-silver  alloys.  Gold 
and  silver  form  an  unbroken  series  of  solid  solutions.  The  series  investi- 
gated consisted  of  ten  members  of  progressively  varying  compositions. 
The  concentration-refractivity  curve  of  the  series  is  continuous  and  shows 

1  Drude,  Ann.  d.  Phys.,  N.  F.,  Vol.  39,  1890,  pp.  481-554. 

2  Willi  Meier,  Ann.  d.  Phys.,  Ser.  4,  Vol.  31,  1910,  pp.  1017-1099. 

3  Bernouilli,  Zeitschr.  d.  Elektro-chem.,  15,  pp.  646-648. 

4  Ann.  d.  Phys.,  Ser.  4,  Vol.  29,  pp.  585  et  seq. 

5  Voigt,  Ann.  d.  Phys.,  Ser.  4,  Vol.  29,  1909,  pp.  956  et  seq. 

6  Littleton,  PHYS.  REV.,  Vol.  32,  1911,  pp.  453  et  seq. 

r  Eckhardt,  Doctor's  Thesis,  University  of  Pennsylvania. 


444337 


157  LOUIS  K.  OPPITZ.  [§S?S 

a  distinct  but  weak  maximum,  while  the  absorptive  index  curve  shows  a 
distinct  minimum  at  about  the  same  concentration.     The  indices  of 
•  •  refraction  of  nearly  all  of  the  gold-silver  alloys  are  higher  than  those  of 
either  component  forming  the  series. 

OBJECT,  THEORY  AND  METHOD  OF  THE  PRESENT  INVESTIGATION. 

The  object  of  the  present  investigation  is  a  study  of  the  optical  con- 
stants of  two  complete  series  of  binary  alloys,  silver-copper  and  platinum- 
silver. 

The  copper  used  in  the  alloys  was  electrolytic  copper,  while  the  silver 
was  1,000  fine  assay  silver. 

The  alloys  were  approximately  of  the  same  size  and  mass.  The  masses 
of  the  metals  constituting  the  alloys  were  carefully  determined  on  a 
chemical  balance.  The  alloys  were  also  weighed  after  being  fused.  In 
no  case  was  there  a  greater  loss  due  to  evaporation  than  one  part  in  about 
three  hundred.  The  boiling  point  of  silver  is  about  1950°  C.  In  order 
to  avoid  the  loss  of  silver  by  evaporation,  the  platinum  was  first  fused, 
and  the  silver  was  introduced  gradually  into  the  melted  platinum.  The 
regulus  was  then  carefully  stirred  by  means  of  a  carbon  rod  and  was  kept 
at  red  heat  for  several  hours,  to  insure  a  homogeneous  mixture. 

The  alloys  were  fused  in  graphite  crucibles  in  a  resistor  furnace.  They 
are  free  from  graphite,  as  is  shown  by  the  values  of  the  optical  constants 
of  the  pure  metals.  The  source  of  energy  was  an  alternating  current 
passed  through  a  step-down  transformer. 

POLISHING  OF  THE  MIRRORS. 

The  method  of  polishing  was  approximately  that  of  Drude.  After 
the  alloy  had  cooled  it  was  mounted  and  a  plane  face  was  turned  on  it  in 
a  jeweler's  lathe.  It  was  then  treated  with  emery.  Fine  grades  of 
French  emery  paper  of  four  degrees  of  fineness  (i.  e.j  o,  oo,  ooo,  oooo) 
were  used.  The  process  of  polishing  began  with  the  use  of  the  o  grade 
that  being  the  coarsest.  The  specimen  was  stroked  in  a  definite  direction 
against  the  emery  paper.  The  emery  paper  was  held  on  a  smooth  plate 
of  plane  glass.  Each  mirror  required  individual  treatment.  The 
pressure  of  the  stroke  was  adapted  to  the  hardness  of  the  particular  alloy. 
The  surface  of  the  alloy  was  stroked  so  as  to  give  to  the  scratches  a  single 
definite  direction.  Then  the  mirror  was  stroked  in  a  direction  at  right 
angles  to  the  scratches  imparted  to  it  by  the  coarsest  grade  of  emery 
paper,  against  an  emery  paper  of  the  next  grade  of  fineness  and  so  on 
until  the  finest  grade  of  emergy  paper  had  been  used.  Each  succeeding 
grade  of  emery  paper  thus  tended  to  remove  or  to  render  less  deep  the 


NoL*2X']  OPTICAL  CONSTANTS  OF  BINARY  ALLOYS.  158 

scratches  introduced  by  the  preceding,  and  to  insure  a  plane  surface.  If 
any  scratches  remained  after  the  finest  emery  had  been  used,  recourse 
was  had  to  a  burnishing  tool  like  that  used  by  silversmiths.  Much  care 
was  exercised  to  keep  the  surface  of  the  emery  paper  free  from  dust  and 
other  forms  of  contamination. 

Drude's  criterion  for  a  satisfactory  optical  surface  was  used:  the  azi- 
muths of  restored  polarization  for  light  parallel  and  perpendicular  to  the 
scratches  must  be  approximately  equal.  The  phase  change  was  found  to 
be  invariable  for  a  given  angle  of  incidence  so  long  as  the  mirror 
remained  free  from  surface  layers. 

OPTICAL  METHODS. 

The  source  of  light  was  a  Bunsen  flame  colored  by  means  of  fused  NaCl. 
This  light  was  filtered  though  an  aqueous  solution  of  K2Cr2O7  which 
rendered  the  resulting  light  practically  that  of  the  D  lines  of  sodium. 
The  light  incident  upon  the  surface  to  be  studied  was  plane  polarized 
at  an  azimuth  of  45°.  This  light  after  reflection  became  elliptically 
polarized  and  was  reconverted  into  plane  polarized  light  by  means  of  a 
Soleil-Babinet  compensator.  The  azimuth  of  restored  plane  polarization 
was  determined  by  means  of  an  analyzing  half-shadow-nicol  system. 
Then  the  analyzing  nicol  was  set  for  extinction  and  the  phase  change 
was  determined  by  the  use  of  the  compensator.  The  angle  at  which 
the  plane  polarized  light  became  incident  upon  the  surface  of  reflection 
was  carefully  determined  by  reading  the  position  of  the  telescope  from  the 
goniometer  circle.  In  order  to  determine  the  azimuth  of  restored  polari- 
zation, a  modified  form  of  the  Zehnder1  half-shadow  polarimeter  was  used. 
This  consisted  of  the  usual  analyzing  nicol  and  a  movable  smoked  glass 
wedge,  adjacent  to  the  nicol  and  moving  over  a  fixed  smoked  glass  wedge. 
In  its  original  form  the  polarimeter  was  made  up  of  an  analyzing  nicol 
adjacent  to  a  fixed  smoked  glass  plate.  The  intensity  of  the  light  used 
for  studying  the  optical  properties  of  the  surfaces  was  found  to  vary  for 
different  angles  of  incidence  and  for  different  optically  reflecting  surfaces. 
It  was  therefore  found  that  relatively  large  angles  of  incidence  were  the 
most  favorable.  At  suggestion  of  Dr.  Eckhardt,  of  this  laboratory,  the 
fixed  smoked  glass  plate  to  which  reference  has  been  made  was  replaced 
by  a  movable  smoked  glass  wedge,  which  could  be  varied  so  as  to  change 
the  length  of  the  path  traversed  by  the  light  passing  through  it.  This 
rendered  it  possible  to  adapt  the  length  of  the  path  to  the  intensity  of  the 
light  traversing  the  polarimeter.  This  gave  half  shadow  equality  through 
a  range  varying  from  7°  to  21°.  The  determination  of  the  position  of 

1  Zehnder,  Ann.  d.  Phys.,  26,  1908,  pp.  985-1018. 


159  LOUIS  K.  OPPITZ. 

extinction  of  the  analyzing  nicol  with  the  polarizer  depended  upon  judging 
half-shadow  equality.  Half-shadow  equality  is  most  easily  judged  when 
the  illumination  through  the  analyzing  nicol  and  smoked  glass  appears 
homogeneous  and  intense.  Two  half-shadow  equality  positions  were 
viewed,  one  on  each  side  of  the  extinction  position  of  the  analyzing 
nicol.  Then  the  analyzing  nicol  half-shadow  device  was  rotated  ap- 
proximately 1  80°  and  two  other  half-shadow  equality  positions  were 
found.  Thus,  there  were  four  readings  in  all  from  which  to  find  the 
extinction  position  of  the  nicol.  The  arithmetical  mean  of  the  positions 
before  and  after  extinction  gives  the  extinction  position. 

Much  practice  was  necessary  for  attaining  proficiency  in  the  judgment 
of  half-shadow  equality.  After  considerable  preliminary  practice,  the 
initial  step  in  the  experimental  work  was  to  determine  the  optical  con- 
stants of  electrolytic  copper.  The  experimental  values  obtained  for  pure 
copper  are  as  follows  : 

«  nK  K 

.640  2.63  4.10  Drude, 

.620  2.57  4.14  L.K.O. 

The  difference  in  the  two  sets  of  values  is  probably  explainable  on  the 
basis  that  the  two  specimens  of  copper  used,  differed  in  purity.  After 
determining  these  optical  constants  for  pure  copper,  those  of  nine  different 
alloys  of  silver-copper,  of  eight  alloys  of  silver-platinum  and  pure  silver 
and  pure  platinum  were  measured.  The  entire  eleven  points  of  the  silver- 
copper  curves  (Fig.  3)  and  the  entire  ten  points  of  the  platinum-silver 
curves  were  experimentally  determined  (Fig.  4). 

In  the  figures,  the  variation  in  the  composition  of  the  alloys  is  ex- 
pressed in  terms  of  the  atomic  per  cent,  of  copper.  The  reflecting  power 
was  obtained  by  calculation,  and  not  by  direct  measurement.  No  ex- 
planation is  at  present  offered  for  the  anomalously  high  reflecting  power 
of  the  silver-copper  alloy  of  4.99  per  cent,  concentration. 

WORKING  FORMULA. 

The  well  known  formulae  of  Drude  were  in  the  calculation  of  the  optical 

constants  : 

w2(i  +  K2}  =  tan2P  sin2  0  tan2  0.  (i) 


The  atomic  per  cent,  of  one  component  is  given  by 

loop 

x  =  -  —  , 

p  +  (100  -  p)  - 


VOL.  X.I 
No.  2.   J 


OPTICAL  CONSTANTS  OF  BINARY  ALLOYS. 


160 


where  p  =   per  cent,  by  weight  of  this-  component,  a  =  its  atomic  weight 
and  b  =  the  atomic  weight  of  the  other  component. 

K  =  tan  Q  (2) 

tan  A  =  sin  Q  tan  2  P  (3) 

cos  2\f/  =  cos  Q  sin  2P  (4) 

h  i  —  2n 


R  = 


where 


n2(i 


(5) 


n  =  the  index  of  refraction, 
K  =  the  absorptive  index, 
A  =  the  phase  change, 
i//  =  the  azimuth  of  restored  polarization, 
R  =  the  reflecting  power, 
0  =  the  angle  of  incidence. 

EXPERIMENTAL  RESULTS. 

Silver- Copper  Alloys. 

Silver  and  copper  form  a  series  of  alloys  in  which  there  are  two  limited 
series  of  solid  solutions,  separated  by  a  gap.     This  gap  consists  of  a  series 
i  of   eutectiferous   alloys.      As   one 

withdraws  from  pure  silver,  silver 
crystals,  i.  e.,  crystal  type  I.  sepa- 
rate out,  and  this  lowers  the  melt- 
ing point.  At  8.5  per  cent,  of 
copper  concentration,  the  solid  so- 
lutions of  silver  are  saturated,  being 
incapable  of  taking  up  any  further 
quantity  of  copper.  After  that,  the 
crystals  contain  varying  amounts 
of  silver  imbedded  in  the  melt. 
At  40  per  cent.,  the  melt  solidifies 
about  the  crystals.  The  saturation 
point  for  copper  is  96  per  cent. 
Likewise  from  100  per  cent,  copper 
to  40  per  cent.,  the  crystals  vary 
in  the  amount  of  copper  contained. 
At  40  per  cent,  of  concentration, 
the  solid  solutions  are  in  equilib- 
rium with  the  melt,  and  therefore  a  eutectic  mixture  is  formed.  These 
thermal  relationships  obtained  from  Guertler's  Metallographie  are  given 
in  Fig.  i. 


Fig.  1. 


161 


LOUIS  K.  OPPITZ. 


The  optical  constants  of  these  alloys  are  shown  in  Table  I.  while  Fig.  3 
is  a  graphical  representation  of  the  same. 

TABLE  I. 

Silver-Copper  Series. 


Wt.  Per  Cent. 
of~Cu. 

Atom.  Per  Cent, 
of  Cu. 

n 

K 

nK 

R 

0 

0 

.202 

17.08 

3.44 

94 

3 

4.99 

.252 

11.35 

2.86 

98.8 

6 

10J27 

.517 

6.51 

3.31 

84.87 

10 

16.49' 

.492 

7.51 

3.69 

87.68 

30 

42.12 

.36 

6.61 

2.40 

80.92 

50 

62,94 

.312 

7.57 

2.37 

82.92 

72 

40.00 

.244 

13.78 

3.36 

93.26 

80 

87.14 

.416 

7.05 

2.93 

84.35 

90 

93.87 

.507 

5.71 

2.90 

80.98 

95 

96.99 

.643 

5.01 

3.22 

80.26 

100 

100.00 

.620 

4.14 

2.57 

73.11 

The  concentration-refractivity  curve  shows  a  minimum  near  the  eutec- 
tic  point,  the  index  of  refraction  being  the  lowest  here  excepting  that  of 
pure  silver.     As  the  eutectic  point 
is  left  in  either  direction,  there  is 
an  increase  in  the  index  of  refrac- 
tion. 

The  absorptive  index-concentra- 
tion curve  shows  a  relative  maxi- 
mum near  eutectic  point,  but  the 
absorptive  index  of  every  alloy  is 
higher  than  of  copper  and  always 
lower  than  of  silver. 

Platinum-Silver  Alloys. 
Similarly  platinum  and  silver 
form  two  series  of  solid  solutions 
separated  by  a  gap.  This  gap  con- 
sists of  a  region  of  heterogeneous 
mixture  of  silver  and  platinum  ex- 
tending from  approximately  34.8 
per  cent,  to  83.5  per  cent,  of  plat-  Fig.  2. 

inum  concentration.    Beyond  these 

points  in  either  direction,  we  find  solid  solutions.     These  relations  are 
found  in  Fig.  2.     This  was  also  obtained  from  Guertler. 


VOL.  X.I 
No.  2.   J 


OPTICAL  CONSTANTS  OF  BINARY  ALLOYS. 


162 


Hf. 


:  Co  flee  nt  r^ion  it  R+omie.  "/,  oj.  Pfr. 


Fig.  3. 


Fig.  4. 


The  optical  constants  of  these  alloys  are  found  in  Table  II.  while  their 
graphical  representation  is  embodied  in  Fig.  4. 


TABLE  II. 

Platinum-Silver  Series. 


Wt.  Per  Cent, 
of  Pt. 

Atom.  Per  Cent. 
ofPt. 

7* 

K 

nK 

R 

Loss  in  Mass  of 
Alloy  After 
Fusing. 

0 
15 

0 
8.9 

.202 
.71 

17.08 
5.92 

3.44 
4.26 

94% 
86.54 

0.000  gr. 
0.035 

30 

19.18 

1.05 

3.69 

3.91 

78.85 

0.020 

40 

26.97 

1.13 

2.736 

3.09 

67.95 

0.032 

45 

31.18 

1.26 

2.42 

3.10 

65.56 

0.000 

48 

33.83 

1.45 

2.29 

3.33 

65.98 

0.000 

50 

35.64 

1.57 

2.15 

3.39 

65.27 

0.000 

62 

47.47 

1.74 

1.82 

3.18 

60.41 

0.000 

90 

83.39 

2.12 

1.85 

3.95 

66.24 

0.000 

100 

100.00 

2.03 

1.96 

3.80 

65.61 

0.000 

Good  working  surfaces  of  the  platinum-silver  alloys  were  easily  ob- 
tained. 

The  concentration  refractive  index  curve  shows  an  unmistakable  in- 
crease toward  pure  platinum.  The  concentration  absorptive  index  curve 
indicates  a  very  sudden  drop  from  pure  silver  to  the  next  member  of  the 
series.  After  that  the  decrease  is  very  gradual.  The  absorptive  index 


I  63  LOUIS  K.  OPPITZ. 

of  pure  platinum  is  slightly  lower  than  that  of  the  solid  solutions  of 
crystal  type  II.  in  Fig.  2. 

In  general,  for  platinum-silver  alloys  as  well  as  silver-copper  alloys 
when  solid  solutions  are  formed,  an  index  of  refraction  which  increases 
with  the  concentration  indicates  a  decreasing  absorptive  index. 

A  typical  sample  of  the  readings  (those  on  the  eutectic  alloy  of  silver 
and  copper)  is  included  below: 

Sample  Series  of  Observations  for  Mirror  No.  I 

Atomic  per  cent.  Cu  =40.  Eutectic  Alloy  of  Silver  and  Copper. 

Angle  of  Incidence  =  74°  7'. 

I.     Scratches  Parallel  to  Plane  of  Incidence. 

Polarizer  at  285°  59'. 

28°  10'  47°  20'  204°  45'  230°  30' 

25  46  50  205    10  35 

10  47   25  204  50  50 

20  10  55  40 

2   20  15  205   00  40 

28   17 47    12  204   56 230  39 

Aver.  37°  44'  Aver.  2 17°  47' 

Polarizer  at  195°  59'. 

295°  35'  327°  55'  121°  40'  142°  35' 

35  328   00  10  45 

296  05  327  40  15  10 

295  40  50  30  30 

40  45  20  30 

295  43 327   51  121    23 142   30 

Aver.  311°  47'  Aver.  131°  56' 

Polarizer  at  105°  59'. 

28°  05'                 47°  20'  204°  45'  230°  45' 

27  55                         05  205   05  25 

28  15          15  204  55  35 
00          25  205  10  40 
15          20                       00       50 

28  06        47  17  204  59       230  39 


Aver.  37°  41'  Aver.  217°  49' 

Polarizer  at  375°  59'. 

296°  05'  328°  05'                   121°  25'  142°  15' 

295  35  327  40  10  50 

296  10  328  00  10  30 

295  35  327  45  15  35 

296  20  328  15  20  40 


295  57 328  15  121  16 142  34 

Aver.  311°  57'  Aver.  131°  55' 

2^  =  217°  47'  -  131°  56'  =  85°  51' 
TT  -  2^  =  311°  47'  -  217°  47'  =  94° 


VOL.  X.I 
No.  2.   J 


OPTICAL  CONSTANTS  OF  BINARY  ALLOYS. 


164 


II.     Scratches  Perpendicular  to  Plane  of  Incidence. 
Polarizer  at  285°  59'. 


28° 

10' 

46° 

50' 

15 

47 

00 

25 

46 

55 

20 

47 

20 

20 

25 

28 

18 

47 

06 

Aver. 

37°  42' 

295° 

15' 

328° 

59' 

30 

327 

40 

20 

50 

45 

45 

40 

55 

295 

30 

327 

50 

Aver. 

311°  40' 

28° 

15' 

46° 

50' 

30 

47 

00 

20 

15 

25 

20 

2 

20 

10 

28 

22 

47 

07 

Aver. 

37°  44' 

295° 

20' 

327° 

35' 

40 

328 

05 

35 

327 

40 

50 

50 

20 

55 

295 

33 

327 

49 

Polarizer  at  195°  59'. 


Polarizer  at  105°  59'. 


Polarizer  at  375°  59'. 


205° 

00' 

230°  30' 

204 

45 

50 

50 

35 

205 

10 

40 

204 

55 

35 

204 

56 

230  38 

Aver. 

217°  47' 

122° 

05' 

141°  55' 

121 

30 

142  10 

40 

30 

15 

20 

20 

20 

121 

24 

142  15 

Aver. 

131°  49' 

204° 

40' 

230°  40' 

50 

45 

55 

35 

205 

10 

40 

00 

50 

204 

55 

230  42 

Aver. 

217°  48' 

121° 

55' 

142°  05' 

25 

141  50 

30 

142  15 

15 

25 

40 

30 

121 

33 

142  13 

Aver.  311°  41' 


Aver.  131°  53' 


=  217°  47'  -  131°  49'  =  85°  58' 


Compensator  Readings. 


Before  Extinction. 

After  Extinction. 

Main  Scale. 

Scale. 

Main  Scale. 

Scale. 

16.00 

04 

17.00 

19 

15.75 

97 

34 

16.00 

48 

47 

14 

77 

09 

86 

Av           16  00 

14 

17.00 

52 

General  Average:  16.50,  33  divisions. 


165  LOUIS  K.  OPPITZ.  HER?ESD 

SUMMARY  OF  RESULTS. 

1.  Near  the  eutectic  point,  the  index  of  refraction  is  lower  than  that 
of  any  other  member  in  the  silver-copper  series,  except  that  of  pure  silver, 
while  the  absorptive  index  is  a  relative  maximum  for  the  same  concen- 
tration. 

2.  The  indices  of  refraction  of  the  alloys  at  the  saturation  points  in 
the  two  regions  of  solid  solutions  for  silver-copper  are  higher  than  that 
of  the  pure  metal  near  these  points. 

3.  From  the  eutectic  point  of  the  silver-copper  series,  there  is  a  marked 
increase  in  the  index  of  refraction  in  either  direction  until  saturated  solid 
solutions  are  formed.     The  absorptive  index  shows  a  behavior  which  is 
approximately  the  inyerse  of  that  shown  by  the  index  of  refraction. 

4.  The  reflecting  power  of  a  metal  of  relatively  low  reflecting  power  is 
in  general  improved  by  mixing  this  metal  with  one  of  relatively  higher 
reflecting  power.     This  is  in  agreement  with  the  work  of  others. 

5.  Whenever  solid  solutions  are  formed  an  increasing  index  of  refraction 
indicates  a  decreasing  index  of  absorption.     This  is  borne  out  by  the 
studies  of  both  the  silver-copper  and  platinum-silver  series. 

In  conclusion,  I  wish  to  record  my  grateful  appreciation  to  Dr.  H.  C. 
Richards  for  placing  at  my  disposal  the  facilities  of  the  Randal  Morgan 
Laboratory  of  Physics.  Not  only  has  he  shown  interest  throughout  the  en- 
tire progress  of  this  work  but  it  is  to  him  that  I  owe  my  first  interest  in  the 
subject  of  optical  constants.  It  is  also  with  pleasure  that  I  acknowledge 
my  indebtedness  to  Dr.  E.  A.  Eckhardt  who  has  aided  me  with  numerous 
valuable  suggestions  in  every  detail  of  the  work. 

THE  RANDAL  MORGAN  LABORATORY  OF  PHYSICS, 
UNIVERSITY  OF  PENNSYLVANIA. 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


